1. Field of the Invention
The present invention relates to three-dimensional graphics and animation, and more particularly, to a system and method for animating a digital facial model based on biomechanical constraints derived from human anatomy.
2. Description of Related Art
Computer graphics (CG) animation is often used in the production of motion pictures and video games to create digital representations of a character's face. In a typical CG animation system, computer software tools are used to create and render virtual objects. The objects may be modified to produce a series of individual frames that are successively displayed in the form of a movie or video file, thereby giving the object the appearance of motion. In the process of creating a CG animation involving a transition of an object from one form to another, a graphics artist will not ordinarily modify the object for every frame. Instead, using a process known as keyframing, the graphics artist creates only the important frames (i.e., keyframes) during which an object changes its size, direction, shape or other properties, and the computer software generates the intermediate frames that form the transition by interpolating between the selected keyframes. The keyframing technique is advantageous in that it significantly reduces the time needed to produce a CG animation.
A drawback of the keyframing technique is that the intermediate frames often appear distorted. These distortions are generally less noticeable when the animated character is non-human (e.g., a robot or cartoon character) in which the audience does not have a preconceived notion as to how the object should appear or move. But, when the animated character is intended to represent a human, the audience will often recognize the distortion of the intermediate frames as not appearing natural. This is particularly noticeable when keyframing is used to animate transitions of a character's face, such as from one facial expression to another (e.g., from a smile to a frown). Because human facial expressions are so familiar, the audience will usually notice and be distracted by slight defects of the animation. Since it is an objective of computer graphics animation to produce realistically appearing visual effects, it is desirable to minimize the distortion of intermediate frames.
Another computer graphics technique is known as motion capture, in which the movement of a real object is mapped onto a computer generated object. In a motion capture system, an actor wears a suit having markers attached at various locations (e.g., small reflective markers attached to the body and limbs) and digital cameras record the movement of the actor from different angles while illuminating the markers. The system then analyzes the images to determine the locations (e.g., as spatial coordinates) and orientation of the markers on the actor's suit in each frame. By tracking the locations of the markers, the system creates a spatial representation of the markers over time and builds a digital representation of the actor in motion. The motion is then applied to a digital representation, which may then be textured and rendered to produce a complete CG representation of the actor and/or performance. This technique has been used by special effects companies to produce incredibly realistic animations in many popular movies.
Motion capture systems are also used to track the motion of facial features of an actor to create a representation of the actor's facial motion and expression (e.g., laughing, crying, smiling, etc.). As with body motion capture, markers are attached to the actor's face and cameras record the actor's expressions. Since facial movement involves relatively small muscles in comparison to the larger muscles involved in body movement, the facial markers are typically much smaller than the corresponding body markers, and the cameras typically have higher resolution than cameras usually used for body motion capture. The cameras are typically aligned in a common plane with physical movement of the actor restricted to keep the cameras focused on the actor's face. The facial motion capture system may be incorporated into a helmet or other implement that is physically attached to the actor so as to uniformly illuminate the facial markers and minimize the degree of relative movement between the camera and face. For this reason, facial motion and body motion are usually captured in separate steps. The captured facial motion data is then combined with captured body motion data later as part of the subsequent animation process.
An advantage of motion capture systems over traditional animation techniques, such as keyframing, is the capability of real-time visualization. The production team can review the spatial representation of the actor's motion in real-time or near real-time, enabling the actor to alter the physical performance in order to capture optimal data. Moreover, motion capture systems detect subtle nuances of physical movement that cannot be easily reproduced using other animation techniques, thereby yielding data that more accurately reflects natural movement. As a result, animation created using source material that was collected using a motion capture system will exhibit a more lifelike appearance.
Notwithstanding these advantages of motion capture systems, the separate capture of facial and body motion often results in animation data that is not truly lifelike. Facial motion and body motion are inextricably linked, such that a facial expression is often enhanced by corresponding body motion. For example, an actor may utilize certain body motion (i.e., body language) to communicate emotions and emphasize corresponding facial expressions, such as using arm waving when talking excitedly or shoulder shrugging when frowning. This linkage between facial motion and body motion is lost when the motions are captured separately, and it is difficult to synchronize these separately captured motions together. When the facial motion and body motion are combined, the resulting animation will often appear noticeably abnormal. Thus, the decoupling of facial and body motion represents a significant deficiency of conventional motion capture systems.
Accordingly, it would be desirable to provide a computer graphics animation system that overcomes these and other drawbacks of the prior art. More specifically, it would be desirable to provide a computer graphics animation system that enables highly realistic animation of a digital facial model.